Cation Disorder and Large Tetragonal Supercell Ordering in the Li-Rich Argyrodite Li7Zn0.5SiS6

Bernhard T Leube; Christopher M Collins; Luke M Daniels; Benjamin B Duff; Yun Dang; Ruiyong Chen; Michael W Gaultois; Troy D Manning; Frédéric Blanc; Matthew S Dyer; John B Claridge; Matthew J Rosseinsky

doi:10.1021/acs.chemmater.2c00320

Cation Disorder and Large Tetragonal Supercell Ordering in the Li-Rich Argyrodite Li₇Zn_0.5SiS₆

Chem Mater. 2022 May 10;34(9):4073-4087. doi: 10.1021/acs.chemmater.2c00320. Epub 2022 Apr 18.

Authors

Bernhard T Leube¹, Christopher M Collins¹, Luke M Daniels¹, Benjamin B Duff^{1

2}, Yun Dang¹, Ruiyong Chen¹, Michael W Gaultois^{1

3}, Troy D Manning¹, Frédéric Blanc^{1

2

3}, Matthew S Dyer^{1

3}, John B Claridge^{1

3}, Matthew J Rosseinsky^{1

3}

Affiliations

¹ Department of Chemistry, University of Liverpool, Crown Street, L69 7ZD Liverpool, United Kindgom.
² Stephenson Institute for Renewable Energy, University of Liverpool, Peach Street, L69 7ZF Liverpool, United Kindgom.
³ Leverhulme Research Centre for Functional Materials Design, Materials Innovation Factory, Oxford Street, L7 3NY Liverpool, United Kindgom.

Abstract

A tetragonal argyrodite with >7 mobile cations, Li₇Zn_0.5SiS₆, is experimentally realized for the first time through solid state synthesis and exploration of the Li-Zn-Si-S phase diagram. The crystal structure of Li₇Zn_0.5SiS₆ was solved ab initio from high-resolution X-ray and neutron powder diffraction data and supported by solid-state NMR. Li₇Zn_0.5SiS₆ adopts a tetragonal I4 structure at room temperature with ordered Li and Zn positions and undergoes a transition above 411.1 K to a higher symmetry disordered F43m structure more typical of Li-containing argyrodites. Simultaneous occupation of four types of Li site (T5, T5a, T2, T4) at high temperature and five types of Li site (T5, T2, T4, T1, and a new trigonal planar T2a position) at room temperature is observed. This combination of sites forms interconnected Li pathways driven by the incorporation of Zn²⁺ into the Li sublattice and enables a range of possible jump processes. Zn²⁺ occupies the 48h T5 site in the high-temperature F43m structure, and a unique ordering pattern emerges in which only a subset of these T5 sites are occupied at room temperature in I4 Li₇Zn_0.5SiS₆. The ionic conductivity, examined via AC impedance spectroscopy and VT-NMR, is 1.0(2) × 10^-7 S cm^-1 at room temperature and 4.3(4) × 10^-4 S cm^-1 at 503 K. The transition between the ordered I4 and disordered F43m structures is associated with a dramatic decrease in activation energy to 0.34(1) eV above 411 K. The incorporation of a small amount of Zn²⁺ exercises dramatic control of Li order in Li₇Zn_0.5SiS₆ yielding a previously unseen distribution of Li sites, expanding our understanding of structure-property relationships in argyrodite materials.